We shall strive especially here below in this document to describe the problems and issues existing in the field of photomultipliers, which the inventors of the present patent application have faced. The invention is of course not limited to this particular field of application but is of interest in any current amplifying technique that has to cope with proximate or similar problems and issues.
A photomultiplier traditionally comprises a photocathode. When an incident photon comes into contact with this photocathode, it releases an electron under photoelectric effect. Such an electron is then directed towards a succession of dynodes in order to be multiplied (through an avalanche effect) so that measurements can be made at the output of the photomultiplier.
More specifically, it is important to be able both to determine the precise instant at which a photon reaches the photocathode (and therefore potentially to put the incident photons in temporal order) and to quantify with precision the energy conveyed by the incident photons.
Indeed, the greater the ability to determine the instant of arrival of a photon with precision, the greater is the ability to determine whether two photons have arrived simultaneously. This criterion is crucial, especially in the medical field where it is sought to identify the annihilation of a positron via the detection of two simultaneously emitted photons that leave a patient, through the use of at least two photomultipliers positioned opposite each other (two photons that arrive at photocathode photomultipliers in such a configuration, with a time lag of the order of one picosecond, are reported as being detected simultaneously).
The quantification of the energy received by a photomultiplier for its part has an impact on the quality of the tomography shots obtained through a PET device. The more precise the quantification, the higher is the quality of the tomography shots obtained. The dynamic range characterizes the ratio of the maximum signal to the minimum signal (often the electronic noise or the single photon) and this ratio is a few thousands.
In the PET field, a search is currently being made for temporal precision levels of some tens of picoseconds (10−12 s) for an activation threshold of some photoelectrons and dynamic ranges of some thousands of photoelectrons. This temporal precision requires bandwidths of the order of one GHz and an amplification of the weakest signals by a factor of about 10 (20 dB). These high bandwidths are now possible at reasonable power levels through advances made in integrated circuits (ASICs), especially in silicon-germanium BiCMOS technology and through advances made with solid-state “silicon photomultipliers”) or MPPC (<<Multi-Pixel Photon Counters>>) which have sufficiently good intrinsic resolutions and limit parasitic inductances. However, their high capacitance (some hundreds of pF) necessitate low input impedance amplifiers, whence the use of current conveyors which enable this characteristic to be obtained.
One difficulty encountered with current conveyors however is that of obtaining high amplification while at the same time appropriately processing the strongest signals which tend to saturate the amplifier and therefore falsify the measurement of amplitude.
There are different types of known techniques in the prior art, enabling these two problems to be resolved simultaneously.
A first technique is described in A. Lucotte and al, “A front-end read-out chip for the OPERA scintillator tracker” in “Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment”, 2004. This article presents a specialized integrated circuit or application-specific integrated circuit (ASIC) positioned at the output of a photomultiplier. Such a circuit has a current preamplifier and gain correction unit comprising transistors and current mirror circuits (aimed at making current copies for separate use), enabling an input current to be amplified by a large factor. More specifically, the current preamplifier and gain correction unit provides two outputs, one output known as a low-gain output and one output known as a high-gain output that respectively that supply a channel called a fast-shaper channel used to obtain temporal information on the incident photons and a channel called a slow-shaper channel that measures the charge of the photon or photons detected by the photomultiplier.
However, one drawback of this first technique is that the “high gain” mirror arm which gets saturated causes distortion in the “low gain” arm and therefore gives an imperfect copy of the current coming from the detector. In addition, these mirrors add parasitic capacitances which, at low power, reduce the bandwidth. Finally, the current copies increase the consumption of the circuit.
A second technique, used in the scintillating tile calorimeter (Tilecal) of the ATLAS detector within the LHC, and described in Z. Ajaltouni and al, “The Tilecal 3-in-1 PMT Base concept and the PMT block assembly”, consists in amplifying not an output current from the photomultiplier, but a voltage (by means of a voltage preamplifier) in having converted the current of the detector in a passive resistor (with a value of generally 50 ohm) The behavior under saturation is then excellent and it is easy to simultaneously deal with both charge and time measuring stages but the signal-to-noise ratio for the weak signals is less good because the 50-ohm resistor dominates the electronic noise. It is then necessary to use a very-low-noise amplifier which typically consumes tens of mW.